The present invention pertains to a method for joining coated metal sheets by means of laser welding in which the metal sheets are placed one on top of another such that the marginal strip of one metal sheet protrudes over the marginal strip of the other metal sheet and the marginal strips are pressed together at least in some areas and are joined by laser welding with a laser beam directed at right angles or essentially at right angles to the plane of the marginal strips. The present invention also pertains to a device for carrying out the method.
Coated metal sheets are used, e.g., for the manufacture and assembly of motor vehicle bodies. The coating material may consist of zinc or the like and has a lower melting point than the metal sheet material. To prevent the coating material from entering the weld pool, it is known that a degassing space originating from the weld seam may be provided. Such a suggestion (DE 38 12 448 C1; DE 198 54 217 A1) goes so far as to use coated metal sheets that are preformed such that they lie one on top of another in the joining area along the weld seam with different radii of curvature and form a degassing space as a result. According to another suggestion (DE 199 32 415 C1), the marginal strips shall have preformed impressions and be pressed together, lying one on top of another, only in the area of these impressions. All this is complicated, because it requires the prior deformation of the coated metal sheets to be joined together.
A method and a device for the laser welding of metal sheets overlapping each other to form a flange seam, in which the welding is brought about by a laser beam located in the plane of the flange seam, while a welding rod is fed, is known from DE 197 05 278 C1. The metal sheets are spread apart here in a position located in front of the laser beam in the welding direction simultaneously with the process, and the laser beam is irradiated into the area of the gap bottom of the spread-apart surface. No degassing space is provided here.
The object of the present invention is to make degassing possible in a simple manner.
According to the present invention a method for joining coated metal sheets by means of laser welding is provided in which the metal sheets are placed one on top of another such that the marginal strip of one metal sheet protrudes over the marginal strip of the other metal sheet and the marginal strips are pressed together at least in some areas and are joined by laser welding with a laser beam directed at right angles or essentially at right angles to the plane of the marginal strips. The protruding marginal strip is pushed away from the other marginal strip during the laser welding to form a degassing space. It is usually sufficient for the protruding marginal strip to be pushed away from the other marginal strip under elastic deformation. A wedge-shaped, acute-angled degassing space is obtained, through which the coating material, which is in the form of a vapor, can escape. Preforming of the metal sheets is not necessary.
According to another aspect of the present invention a device for carrying out the method is provided with a support for the two marginal strips placed one on top of another and with a pressing piece that can be moved against and toward the support.
The device has a holding-down device that can be moved in relation to the protruding marginal strip, arranged at or next to the pressing piece. If the pressing piece is lowered onto the marginal strip of the metal sheets to be joined together, which marginal strip is arranged on the support, and the marginal strips are clamped as a result, the protruding marginal strip is also pushed away at the same time from the other marginal strip while the degassing space is formed. The holding-down device is connected for this purpose to the pressing piece and is arranged in the area of the projection of the marginal strip. It is recommended that the holding-down device be arranged adjustably at the pressing piece in order for the deflection of the protruding marginal strip to be able to be adapted to the particular conditions prevailing during the operation.
The pressing piece and/or the support may have at least one opening for a focused laser beam, depending on whether the laser beam shall be directed toward the metal sheet from the top or from the bottom. To make welding through possible, the support or the pressing piece may have a recess, which corresponds to the opening in the other component. The pressing piece and/or the support may also be cooled. Furthermore, a gas feed line opening into the opening may be provided.
In another embodiment, the pressing piece is a roller acting on the narrower marginal strip, and a holding-down roller acting on the projection of the protruding marginal strip is provided. If the rollers have different diameters, they may be arranged coaxially. The rollers may be preferably arranged on a movable component (e.g., a robot) with a laser welding means, in which case the movable component must also generate the necessary pressing pressure between the roller and the support.
Exemplary embodiments of the present invention, which are shown in the drawings, will be explained below. The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated.